The present invention relates to compositions and devices which emit radiation through the employment of a first substance functioning as a source of an exciting radiation and a second substance which interacts with the exciting radiation to provide an emitting radiation. In particular, the present invention relates to luminescent compositions and devices, especially radioluminescent compositions and devices.
Compositions providing radioluminescence are well known and are, for example, used as alternatives for conventional electric light sources. Radioluminescent lamps have been employed in such applications as safety lighting, emergency signs (e.g., exit signs), airport runway lights, background lighting for aircraft and space applications, and other applications where electrical light is either difficult or impossible.
Conventional radioluminescent lamps comprise a phosphor powder which is deposited on the inside surface of a hollow glass tube. Phosphoric acid or an organic binder is used to adhere the phosphor to the tube surface. The hollow glass tube is then evacuated and backfilled with a beta-emitting radioisotope, usually tritium gas. Beta particles produced by the radioactive decay of tritium atoms impinge upon the phosphor resulting in the release of energy in the form of light. See, e.g., U.S. Pat. No. 4,855,879 (Soltani et al.).
Unfortunately, while radioluminescent lamps are very useful for the above-described applications, their maximum light output is somewhat limited. Two effects contribute to the limitation of maximum light output for radioluminescent light sources. Firstly, based on calculations, the saturation power flux predicted for pure tritium gas at 1 atmosphere (the typically used radioisotope) is only about 11 microwatts/cm.sup.2. Tritium has a very low beta particle energy, i.e., E.sub.ave =6 keV and E.sub.cutoff =18.6 keV. Due to this low beta particle energy, self-absorption of the beta energy by the tritium gas itself becomes significant. For this reason, the beta particles that excite the phosphor deposit on the inside of the glass tube only can come from a limited gas thickness. Based on computer simulations, the predicted gas thickness is about 2 cm for 1 atmosphere of pure tritium gas. In conventional use, a tritium thickness of about 0.3 cm is typically used with 1.3 atmospheres of tritium in order to optimize the light output per curie of tritium gas used.
Secondly, due to the low average beta energy, beta particle penetration into the phosphor particles is limited to about 1-20 .mu.m. Even though phosphor materials are very reflective, as a result of their high refractive index the phosphor layer deposits on the inside of the glass quickly become opaque to light even for deposits of only a few particles thick. For these reasons, increasing the thickness of the phosphor deposit on the inside layer of the tube does not result in an increase in brightness for a typical 1 atmosphere gas tube.
Moreover, there are important health and safety concerns which go hand-in-hand with the use of radioactive materials. Since radioluminescent lamps typically employ tritium gas as a radioisotope, there is much concern regarding emissions of tritium gas either by leakage or due to breakage of the hollow glass tube.
Another practical application of radioisotopes and luminescent materials is their use in nuclear or atomic batteries and photovoltaic generators. See, for example, Olsen et al. (U.S. Pat. No. 3,706,893) and McKlveen et al., "Radioisotope-Powered Photovoltaic Generator," Nuclear Technology 43:366-372 (May 1979).
For additional discussion on luminescent materials (e.g., phosphors and luminescent glasses) and their uses, see J .B. Birks et al., Scintillation Counters, McGraw-Hill Book Co., Inc., 1953; P. Goldberg (editor), Lumination of Inorganic Solvents, "Cathodoluminescence", pp. 151-184, 1966; H. W. Leverenz et al., Luminescent Materials, Vol. 10, Jul., 1939, pp. 479-493; H. W. Leverenz, "Cathodoluminescence as Applied in Television", RCA Manufacturing Co., Inc., Harrison, N.J., pp. 131-175; H. W. Leverenz, "Phosphors Versus the Periodic System of the Elements", Proceedings of the I.R.E., May, 1944, pp. 256-263; C. Feldman, "Development and Applications of Transparent Cathode-Ray Screens", Journal of the S.M.P.T.E., Vol. 67, Jul., 1958, pp. 455-460; G. W. Ludwig et al., "The Efficiency of Cathode-Ray Phosphors", J. Electrochem. Soc., Vol. 117, No. 3, Mar., 1970, pp. 348-353; and, J. D. Kingsley et al., "The Efficiency of Cathode-Ray Phosphors", J. Electrochem. Soc., Vol. 117, No. 3, Mar., 1970, pp. 353-359.
Related copending patent application Ser. No. 07/435,092, filed Nov. 13, 1989 (Clough et al.), hereby incorporated by reference, discloses radioluminescent compositions comprising a zeolite crystalline material in which is sorbed a tritium-containing component and at least one luminophore. The loaded zeolite can be suspended in an optically clear polymer or optically clear silica matrix (e.g., silica gel). Another disclosed embodiment comprises an optically clear polymer matrix, a soluble tritiated organic component containing olefinic or alkynylic bonds prior to tritiation and an organic scintillation dye for transferring primary scintillation to a red shifted emission.